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Choosing the Proper PCC Mix for Durability

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A presentation given during the 39th Annual NCPA Concrete Paving Workshop. By Dale Harrington, P.E., CP Tech Center

Published in: Engineering
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Choosing the Proper PCC Mix for Durability

  1. 1. 1 CHOOSING THE PROPER PCC MIX FOR DURABILITY 39th Annual Concrete Paving Workshop, Lincoln, Nebraska January 16, 2018 Dale S. Harrington P.E. -Representing the CP Tech Center
  2. 2. 2 What Properties from Mix are Needed 1. Strength 2. Workability 3. Crack Resistance 4. Freeze Thaw Resistance 5. Deicing Chemical Resistance Assuming we have quality aggregates and quality cementitious materials
  3. 3. 3 Strength • Standard mixes are adequate to obtain proper strength gain in a reasonable time. • Exceptions may be special circumstances where early opening is required. ─ IDOT maintenance mixes provide for the early openings but have increased potential for dry shrinkage ─ Standard mixes with excess accelerator admixtures can result in cracking before finishing and/or saw cutting can be completed.
  4. 4. 4 Adding Cement Goes so Far with Strength
  5. 5. 5 Strength Gain Cementitious Material Properties Silica Alumina Calcium Oxide Sulfate Type I Cement 22% 5% 65% 1% Class F 52% 23% 5% 0.80% Class C 35% 18% 21% 4.10% Slag 35% 12% 40% 9% SCM’s can can slow initial strength gain but improves long term strength Silica – Ties up alkalies to help control ASR Alumina – Set off right away and to control flash set use gypsum (sulfate) Calcium Oxide –Provides faster set possibilities Sulfates – Help control aluminates to prevent permanent hardening (flash set). Too much sulfate causes false set but can be overcome.
  6. 6. 6 Workability • Important property of fresh concrete • Amount of mechanical work required to place and consolidate the concrete • Provides adequate consolidation and reduced entrapped trapped air voids • Proper aggregate gradation greatly improves workability ─ Proper filling of voids between larger particles
  7. 7. 7 Aggregate Moisture States Absorption is a function of the amount of void space in the aggregate
  8. 8. Aggregate Gradation • Should be well-graded • Control combined grading to increase amount of aggregate in the mix – Reduced paste (shrinkage, heat, cost) Aggregate is inexpensive and a good filler
  9. 9. 9 Aggregates Gradation • Most of the volume of a mixture • Influences: – Strength – Workability – Durability
  10. 10. 10 Crack Resistance • Use Type I/II cements (avoid high early strength except special conditions) – Minimize shrinkage – Helps with slab warping • Use SCM to reduce thermal shrinkage • Use low CTE aggregates – Minimize curvature & stress – Helps prevent aggregate expansion
  11. 11. 11 DURABILITY
  12. 12. 12 Durability Some joint distress have appeared in the last 10 years Primary factors causing distress: 1) Reaction between certain deicers in the matrix of concrete 2) Lack of proper air entrainment; freeze-thaw damage 3) Saturation (permeability);freeze-thaw damage
  13. 13. 13 How to Achieve Durability The Right Mix for Durable Concrete Low w/c ( low permeability) Freeze-Thaw Resistance Use of SCM’s Deicing Salt Resistance Proper Entrained Air /Lower Saturation Freeze-Thaw Resistance
  14. 14. 14 Air Entrainment • Vinsol / Resin / Tall Oil / Synthetics • Air Void System – Spacing factor <0.008 inch – Air content >5% behind the paver – SAM
  15. 15. 15 Super Air Meter (SAM) • Over 90% of test mixes, a SAM number of 0.20 has been shown to correctly determine whether the spacing factor is above or below the 0.008 inch.
  16. 16. 16 Lower Water/ Cement Ratio • w/c 0.42 • w/c 0.60 – dark voids where water once occupied space – left pores Low W/C= Low Permeability = Less Saturation= Improve Durability
  17. 17. 17 • Watch the amount of added water! • Added water not to exceed max w/c (check batch tickets) • If water added, mix for additional 30 revolutions Water
  18. 18. 18 Water & Permeability • More water – means more space between cement grains
  19. 19. 19 Effects of Extra Water on Concrete • Adding 1 gallon / yd3 • Increases workability ~1” • Lowers strength ~200 psi • Increases drying shrinkage ~10% • Increases permeability ~ 50%
  20. 20. 20 Low Permeability Fill First = Gels & Capillary Pores fill first ( 10-18 hours) Fill Last = Entrained and Entrapped Air (Months to years) If undergoes Freeze- Thaw in critical saturated state damage will occur within a few cycles regardless of air volume Weiss 2014
  21. 21. 21 Sizes of Concrete Components
  22. 22. 22 First – Filling of Smallest Pores -Gel and Capillary Pores- (10 to 18 hours) Air Entrainment Capillary Pores Gel Pores Entrapped Air Weiss 2014
  23. 23. 23 Second- Filling of Larger Pores -Entrained and Entrapped Air- (months to years) Weiss 2014 Air Entrainment Capillary Pores Gel Pores Entrapped Air
  24. 24. 24 Critical Saturation Rates Weiss 2014
  25. 25. 25 High Concentrations of Deicers • High concentration of magnesium and or calcium chlorides (deicers) can react with cementitious matrices. • This results in expansive deposits that can lead to diminished durability.
  26. 26. 26 Impact on Joints • These types of deicers can be more effective for ice removal • Even when low concentration of deicers are applied to the pavement, evaporation that occurs during drying cycles will eventually produce a highly concentrated deicing solution • At some point these levels will reach a level of super saturation that affects mineral deposits in the concrete
  27. 27. 27 Impact on Joints • The formation of Calcium Silicate Hydrate (C-S-H) and Calcium Hydroxide (CH) are the two principal ingredients that mesh into a solid mass forming concrete pavement. • Magnesium and calcium chloride will react with CH with water at between 32 F and 122 F, depending on the salt concentration.
  28. 28. 28 Impact on Joints • This reaction results in the formation of calcium oxychloride which results in flaking (expansion) of the hardened paste causing significant damage particularly in joints. • Oxychloride expansion can be 3 times greater than freeze-thaw expansion. • The use of SCM’s ( fly ash, slag, and silica fume) has shown to reduce the formation of calcium oxychlorides by tying up CH. • Use of sealers has also shown the potential to limit the interaction between salts and CH by reducing exposure.
  29. 29. 29 Temperature for Calcium Oxychloride Formation (when hydrated cement paste is brought into contact with calcium chloride solution) Weiss 2014
  30. 30. Cement Water SCM + Water + + C-S-H + CH = more C-S-H = How Do Also SCMs Work?
  31. 31. 31 Putting It Together Engineer the Mix Engineer the Deicing Chemical Application Engineer the Drainage Concrete Durability Approach
  32. 32. 32 Class C & Class C-SUD Class C-4 Mix Cement 0.118 Water 0.159 Air 0.06 Fine Aggr. 0.331 Coarse Aggr. 0.332 Cement 0.106 Water 0.133 Air 0.06 Fine Aggr. 0.315 Coarse Aggr. 0.386 C Mix Class C-SUD Mix C-SUD Mix 16% 13%
  33. 33. 33 Conventional PCC – M Mix M-Mixes High early strength for patching Basic Absolute Volumes of Materials Per Unit Volume of Concrete More cement than C mixes
  34. 34. 34 Rural Durability Mixes (QM-C) (by Contractor) • Quality Management Concrete (QM-C) mix – Improved placement characteristics (workability) for slip form only – Improved workability = improved durability – Required on Iowa DOT projects > 50,000 SY – Three gradation aggregate (typ. central plant not ready mix) – Ideal for large mainline rural paving projects – Not ideal for small, urban or extensive staging projects – Basic w/cm ratio is 0.40 – Max. w/cm ratio is 0.42.
  35. 35. 35 Questions www.cptechcenter.org

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